1. Field of the Invention
This invention relates to an improvement in one type of printing apparatus for printing characters on paper and the like. Specifically, the invention is an improved matrix printer.
2. Description of the Prior Art
Matrix printers, as is known to those skilled in the art, form characters by printing a plurality of dots on a printing medium. The dots are printed by impacting the ends of selected print wires against a ribbon and the printing medium. The ends of the print wires are arranged in a substantially 2-dimensional array which is positioned opposite a platen which supports the printing medium. The array is movable laterally along the print line to form selected characters by selective sequential activation of print wires, thereby producing words and other indicia and thereby providing a line of print. The array and the activation components associated with each print wire are located in a unit called the matrix printing head.
Prior art matrix printers activate a selected print wire end by use of a solenoid (generally in conjunction with a pivoting lever arm) which, upon energization, causes the wire to advance longitudinally in its guide toward the platen. In certain prior art systems the solenoid plunger is moved, while in others the plunger is stationary and an associated armature or lever arm is caused to move by attraction to the solenoid plunger. The print wire is pivotably attached to a lever arm in both cases; it is the lever arm being attracted to the plunger, or the plunger attracting or hitting the lever arm, that causes motion of the print wire. In either case, the wire is typically retracted to its normal or rest position by a spring. After repeated use, the movement of the print wires is impeded by increased friction in the guides due to wear, paper dust, etc. The solenoids, generating a fixed magnetic force for a given current input, are unable to overcome this increased friction and therefore cannot maintain proper print wire impact force without an increase in electrical current input to the solenoid coil. Consequently, the print wires move slower and with less impact, thereby causing a deterioration in printing quality. Furthermore, the decreased print wire impact force affects the time and speed of print wire recovery (duty cycle) since the wire is no longer retracted with the same reaction force (i.e. bouncing off the platen) as in a new system, even though a return spring is utilized.
Moreover, to provide proper print wire impact force, relatively large solenoids have been used in prior art matrix printers. Since such printers operate essentially on principles of magnetic attraction, many coils of wire and heavy and massive solenoid plungers were required in such prior art matrix printers. Accordingly, prior art matrix printing heads were relatively massive structures with high inertia, and drive systems for laterally moving the printing head along the print line necessarily had to be larger, stronger, and naturally more costly. Massive printing heads of solenoid type matrix printers also limited printing speed by inhibiting lateral movement along the print line. Furthermore, prior art solenoid type matrix printers utilizing moving plungers also must overcome the inertial mass and consequently low acceleration characteristics of the plunger, thus further limiting maximum available printing speed. Also, to improve efficiency and increase mechanical advantage, solenoids are generally used with pivoted lever arms to provide a compromise between overall matrix printing head size and weight and the impact force that may be generated for a given electrical power input. The resulting prior art arrangement unfortunately still has an undesirably large size and weight and undesirable mechanical complexity.
Furthermore, solenoids require a relatively large amount of current to generate a suitable print wire impact force. That is, as is well-known to those skilled in the art, the magnetic field induced by the solenoid coil in the solenoid plunger must be of sufficient magnitude to rapidly attract a lever arm to the plunger (or vice versa). The magnitude of the field determines the rapidity of movement of the print wire and consequently determines the impact force of the print wire on the printing medium. A relatively large current is needed to produce the desired magnetic field, and this mandates relatively large, heavy and costly electrical power supplies and other components.
Another inherent deficiency of prior art solenoid type matrix printers is their limitation of printing speed due to temperature effects. Solenoid characteristics restrict the speed with which magnetic flux may be built up to a sufficient magnitude to produce acceptable print wire impact forces. As is well-known to those skilled in the art, there is a tradeoff between speed and solenoid temperature rise since the faster a solenoid is cycled on and off, i.e. the faster the printing speed, the greater the temperature rise within the solenoid.
Additionally, prior art matrix printers generally have the activating solenoids arranged radially about a central axis. Generally, a plurality of solenoids are arranged in a plane and the diameter of the essentially cylindrical structure thus formed is relatively large, depending upon several parameters, one of which is the number of print wires desired in the matrix printer. The size of this matrix printing head restricts visibility of the printed characters. While this matrix printing head size does not render such a printer objectionable as a computer printer, it does render the matrix printer unsuitable as a consumer-oriented product such as a typewriter.
It is apparent that the aforementioned unavoidable inherent deficiencies of solenoid type matrix printers generally necessitate larger, stronger and costlier ancillary elements such as larger power supplies and other components necessary for lateral movement of the carriage holding the matrix printing head.
Accordingly, there is a need in the matrix printer art for a matrix printing head capable of rapidly printing characters with an acceptable print wire impact force while being simultaneously efficient and lightweight in order to simplify the supply of power to the matrix printing head, particularly, for a larger number of print wires, e.g., more than 14 wires.
While a prior printing apparatus has been discovered utilizing an operating principle similar to the present invention, that prior art printer, as disclosed in U.S. Pat. No. 3,087,421, issued Apr. 30, 1963 and entitled "High Speed Printer" was not a matrix printer. In any event, the disclosed printing apparatus did not employ print wires and did not utilize several elements of the subject invention, as will be more clearly understood below.